The present invention relates in general flow meters and in particular to a method of and a device for a magnetically inductive measurement of rate of flow of an electrically conductive medium flowing through an electrically insulating pipe section. The medium is exposed to a periodically pulsating electromagnetic field generated by an exciter coil energized by pulsating direct current. The induced useful voltage component which is proportional to the speed of flow, together with interference voltage components are detected during each pulse of the excitation current over a certain scanning period by two electrodes arranged on the insulating pipe section. In a convertor the detected voltages are integrated and the integrated values are mutually compared to eliminate interference alternating current and direct current voltage components.
In the prior art method of such an electromagnetically inductive flow rate measurement the unavoidable alternating current voltage components which usually occur at main frequency of 50 or 60 hertz, are integrated to zero during each half period of the supply current because the constant detection or integration time interval of respective measuring voltages is selected to correspond to the period of the interfering alternating voltage components or to an integral multiple thereof. During the integration time interval which matches the duration of a period of the alternating current interference voltage, the positive and negative half waves of the interference voltage cancel each other and consequently the average value of the interference voltage components which is superposed to the useful voltage components is zero.
After the elimination of the alternating current interference voltage components U.sub.s the integration values resulting from the detection of the measuring voltage U during the positive and negative half waves of the supply current, still contain a direct current interference voltage components U.sub.g. In a known method of this kind, the direct current interference voltage components is neutralized in a second step of the method in which the difference between the positive and negative integration values is made resulting in a pure useful voltage U.sub.n.
This known measuring method using pulsating direct current field has the disadvantage that rapid changes in the flow rate or pulsating flows cannot be correctly determined and consequently considerable measuring errors may occur. Since in the case of an interference voltage of a frequency of 50 hertz and two detections of the measuring voltage per a period of the supply voltage the detection for a complete interference voltage period requires 40 milliseconds (80 milliseconds for two such periods) and additional time is lost for the built-up of the electromagnetic field during switching on or switching over of the excitation current, prior art flow rate measurements based on magnetic induction operate in practice with electromagnetic fields excited at freuqencies from 81/3 hertz to 31/8 hertz. The time spacing between two delivered measured values of the throughflow amount in this case between 120 to 320 milliseconds and that time between two detections thus increases to 60 up to 160 milliseconds. Sudden drops or rises in the flow rate or flow speed of the measured medium may frequently occur in substantially shorter time intervals and accordingly the number of detections per second, the detection rate in such known measuring methods using switched-on field is too small for achieving an accurate measurement.
Also in the case of momentary flow rate measurements occuring for example in dosing, the measuring error increases proportionally with decreasing dosing or metering interval. For instance, the metering or dosing times in charging beverages, liquid medicines or liquid food stuffs into containers takes presently at least in part below a second and consequently without sufficient number of measuring values per dosing cycle no accurate metering is possible.
Moreover, in prior art magnetically inductive flow rate metering methods using closed or switched on field, the starting period of the excitation current and hence the power consumption of the apparatus is high, requiring considerable amounts of electric energy for the excitation of the coil and for the buildup of the magnetic field.